1. Field of the Invention
This invention relates in general to subsea wells, and in particular to a means for monitoring the annulus pressure between a production casing string and an intermediate casing string.
2. Description of the Prior Art
Subsea wells of the type concerned herein will have an outer wellhead housing secured to a string of conductor pipe which extends to a first depth. A high pressure, or inner wellhead housing lands in the outer wellhead housing. The high pressure housing is secured to an outer string of casing, which extends through the conductor. An intermediate string of casing extends through the outer casing. The intermediate string of casing is supported at the upper end by a lower casing hangar which lands in the inner wellhead housing. A production or inner string of casing extends through the intermediate string of casing to the final depth. The inner string of casing is supported by an upper casing hanger which lands in the wellhead housing above the lower casing hanger. Both of the lower and upper casing hangers are sealed separately by casing hanger seals to the wellhead housing.
A tubing hanger assembly lands in the upper casing hanger. The tubing hanger assembly secures to a string of tubing that extends through the inner casing for producing well fluids. A tree assembly lands on the inner wellhead housing. The tree has valves and various controls for controlling the well production. The tubing hanger has a production flow passage through it for the production fluids and normally another passage for communication with the annulus surrounding the tubing.
A sealed annulus locates between the inner string of casing and the intermediate string of casing. Normally there will be no pressure in the annulus between the inner and intermediate strings of casing because production will be through perforations in the inner string of casing, and pressure in the in the lower portion of the inner string of casing is normally sealed by a packer. If pressure increased within this annulus, it would indicate that a leak exists in one of the strings of casing. The leak could be from several places. Possibly, during testing, the upper casing hanger seal leaked when pressure was applied to the wellhead housing. Possibly the packer or inner string of casing above the packer developed a leak. Possibly, the cement around the inner string of casing leaked. Regardless of the location, this leak could result in high pressure buildup in the annulus surrounding the inner string, which is not wanted. If high pressure resulted from a wellhead test, the high pressure could collapse a portion of the inner string. Currently, there are no techniques used to monitor the annulus pressure surrounding the inner string of casing.